1. Field of the Invention
This invention relates to a digital information recording apparatus adapted to record digital information onto an information recording medium, such as a DVD-R (DVD-Recordable), which enables recording of approximately 7 times higher in density than a conventional CD-R (Compact Disk-Recordable).
2. Description of the Prior Art
Recently, there has been broadly researched and developed a so-called DVD as an information recording medium having the recording capacity of approximately 7 times higher than a conventional CD, and a DVD-R, which is a write-once type DVD, has also been researched and developed.
The DVD-R uses, as a recording layer, a dye film on which recording tracks (grooves (guide grooves) or lands) are formed in advance. In recording, a micro-size portion (where a recording pit will be formed) on the recording track is irradiated with the recording laser light, modulated according to digital information to be recorded and appropriately focused with respect to the micro-size portion, to heat the dye film at the micro-size portion due to the energy conversion into the heat energy. By this, the characteristic, such as the reflectivity, of the dye film at the micro-size portion is changed to form the recording pits. In reproduction, the digital information recorded is reproduced by using the difference in characteristic, such as the reflectivity, against the reproduction laser light of the recording pit portion and other portion where no recording laser light has been irradiated. In order to achieve the above mentioned high recording capacity in DVD-R, the information pits should be formed such that the length of the pit in the radial direction of the disc (i.e., pit width) is approximately 0.4 .mu.m, the length of the pit in the circumferential direction (i.e., pit length) is approximately 0.4 .mu.m for the minimum length pit and approximately 1.9 .mu.m for the maximum length pit.
In the case of the conventional CD-R using the above mentioned dye film, there may be such a problem that the reproduction waveform is distorted if the recording pits are recorded by the laser light modulated according to the digital information to be recorded and then they are reproduced. One cause of this distortion is that the shape of the recording pit at front and rear portions are asymmetric in the circumferential direction of the CD-R, i.e., the pit shape is thin at its front portion and thick at its end portion, forming a tear-like shape, thereby introducing the distortion. Specifically, referring to FIGS. 1A to 3A, if the information is recorded by the laser light modulated to have the waveform shown in FIG. 1A, the temperature distribution of the area of the dye film irradiated with the laser light includes the lower temperature area at the front portion and the higher temperature area at the rear portion due to the heat storage effect as shown in FIG. 1B. As a result, the recording pits of tear-drop shape shown in FIG. 1C are formed.
Another factor which deteriorates the symmetricalness of the recording pit shape is that the light spot formed at the laser light irradiation position covers beyond the area where the recording pit should be formed. Namely, when the laser light is irradiated, the light spot covers the area other than the pit recording area so that the recording pit formed becomes asymmetrical at its front and rear portions in relation to the digital information to be recorded.
In order to overcome this problem, the conventional CD-R does not modulate the laser light by the modulation signal obtained by simply modulating the pulse signal by the digital signal to be recorded. Namely, the pulse signal is modulated by the digital signal to be recorded, and then the waveform of the signal thus modulated is then waveform-converted to obtain the recording signal corresponding to the digital signal to be recorded. Thereafter, the laser light is modulated by the recording signal and the recording pits are formed using the thus modulated laser light.
Specifically, as shown in FIG. 2, the modulated signal is further waveform-converted such that a certain time period of the recording signal, corresponding to the front portion of the recording pit, is set to zero and the following period of the recording signal includes successive short-period pulses. In the example shown in FIG. 2, for the modulated signal indicating high level for 11T period, the initial potion of the recording signal is set to zero for the time period 1.5T, and the later half of the recording signal is waveform-converted to include successive pulses of 0.5T length, thereby producing the recording signal as shown. Then, the laser light is modulated by this recording signal and then irradiated on the dye film. Here, "T" represents the time period corresponding to a single reference clock period in the digital signal to be recorded. In the case of DVD-R, it is standardized that the digital signal to be recorded is constituted by the combination of 12 kinds of data pulses having the pulse lengths from 3T to 14T and the sync pulse of 14T length. By modulating the laser light with the waveform-modulated recording signal, it may be possible to avoid the undesirable temperature distribution on the dye film as shown in FIG. 1B and to form the oval recording pits having desired symmetricalness corresponding to the digital signal to be recorded. Referring to the waveform shown in FIG. 2, the initial pulse having 1.5T length will be hereinafter referred to as "top pulse (TP)" and the successive pulses of 0.5T length, following the top pulse, will be hereinafter referred to as "pulse train (PT)".
In the conventional CD-R, in order to waveform-convert the modulated signal into the recording signal as shown in FIG. 2, analog type delay lines (e.g., configured by concentrated constant elements, distributed constant elements, or active element such as logic gates) and AND circuits or flop-flop circuits are mainly employed. In the conventional CD-R, the reference clock period is about 230 nsec, and the accuracy of about 10 nsec is necessary and sufficient for the waveform conversion of the modulated signal into the recording signal. Therefore, the above mentioned delay lines can be used to configure the waveform conversion circuit.
On the contrary, the reference clock period for DVD-R is set to 37 nsec to achieve the high recording capacity, and it is further required that the accuracy of the waveform conversion is very high. Specifically, the resolution and reproducibility of about some nano seconds (1 nsec if possible) is required. It is further necessary to enable various setting change (such as a change of converted waveform) with such accuracy being maintained.
However, the above mentioned delay lines available have the resolution of about 5 nsec. Further, it is necessary to account for the error in accuracy and/or variation during use of about .+-.10% of the total delay time. Further, accounting for the irregularity of delay between the connections connecting the respective elements, the substantive accuracy is further deteriorated. In a configuration with variable delay time, the serial connection of special high-resolution delay lines should be employed, and total resolution is greatly deteriorated due to the irregularity of the respective delay lines.